The present invention relates generally to a fuel injection system of engines of automotive vehicles and more particularly to a fuel injection control method and device capable of controlling the flow rate of fuel and air induced into an engine. Additionally, the present invention relates to the ignition timing control in a manner to maintain optimum values thereof over a wide range of engine operational conditions.
Fuel injection systems employing air flow meters have been used in various kinds of automotive engines. In a typical system of known type, the air flow meter is installed in the air intake system at an upstream position of the throttle valve to detect accurately the flow rate Q of the air induced into the engine. Then the basic fuel injection quantity T.sub.P corresponding to the fuel injection duration such as to provide a fuel quantity corresponding to the induced air flow rate Q. For example, the basic fuel injection quantity T.sub.P which is close to the theoretical (ideal) air-fuel ratio A/F is calculated in the formula of T.sub.P =Q/N, where N is the engine speed. The fuel injector is basically controlled on the basis of T.sub.P.
Conventionally, the T.sub.P is used as data for determining load in the ignition timing calculation. Therefore, a high degree of accuracy is required in the measurement of the engine induced air flow rate Q. Accordingly, precise means such as air flow meters of the hot-wire type possessing high response are used.
However, as described above, the air flow meter in such systems has conventionally been installed upstream of the throttle valve and therefore has not been positioned to directly measure the flow rate of the air induced into the engine. For this reason, in the case of a transient state of the engine operation such as the throttle valve is opened rapidly from a closed position to a wide opening, the quantity of the air flowing into the engine increases. At the same time, the pressure in the collector chamber or the intake manifold of the intake system installed downstream from the throttle valve also rises. Therefore, this excessive quantity of air mass induced by this pressure rise is also measured by the air flow meter.
That is, in the excessive quantity of air flowing into the engine is measured with the air flow meter on the upstream side of the throttle valve, when the throttle valve is opened widely. This excessive air quantity becomes a spike in the air intake system. The larger the capacity of the collector chamber or intake manifold is, the larger the spike is. Furthermore, this spike quantity increases with increase of sensitivity of the air flow meter.
Then, as the fuel injector is installed downstream from the intake manifold, excessive fuel is supplied into the engine according to the air quantity measured by the air flow meter. Consequently, an abrupt enrichment of the air-fuel ratio occurs, then the contents of harmful gases such as CO and HC in the exhaust gas increase. In the worst case, the engine power drops because of the over-rich mixture, and the drive feeling deteriorates.
Furthermore, if the controlling system also includes the ignition timing control, there is the possibility of an instantaneous retardation of the ignition timing caused by the functioning of a knocking prevention mechanism. As a consequence, an instant drop of the engine power and a deterioration of the emitted gases occur.
At transient state of the engine operation, deviations of the air-fuel ratio and the ignition timing from their optimum values also occur when the opening degree of the throttle valve varies from a widely opened position to the closed position.
Accordingly, there has previously been proposed an engine control system for controlling the fuel injection quantity and ignition timing which includes the following operational functions as disclosed in Japanese Patent Laid-Open Publication No. 261645/1987 by the same invention. With the use of a model of the air within the intake pipe, the pressure within the intake pipe is estimated according to the detected air flow rate measured by the air flow meter, the detected opening degree of the throttle valve, and the detected engine speed. Then, from this estimated pressure, the quantity of air filling the collector chamber and the intake manifold downstream of the throttle valve under a transient condition is estimated. From this estimated air quantity and the above mentioned air flow rate measured by the air flow meter, the actual induced air quantity is determined. Then, from this actual induced air quantity and the engine speed, the fuel injection rate and the ignition timing are determined.
However, as this control system requires a large number of measurement factors and moreover involves complicated computation formulae, the microcomputer memory capacitor becomes large.